Many optical experiments and optical based equipment require some form of modulation of the linear polarization of a light source. Variable modulators exist in three varieties.
The most widely used modulator is based on liquid crystals, with elongated molecules in a liquid oriented by an electric field. The retardation or rotation of a laser beam transmitted through this liquid crystal layer depends on this orientation and can thus be modulated by an electric field. Modulation can be done in the order of 0-10 kHz, for a narrow wavelength range.
Stress induced birefringence or the piezo-optic effect is used in photoelastic modulators, which can operate at high frequency (100 kHz) but also only for a narrow wavelength range.
A simpler approach involves the mechanical rotation of a fixed retarder. This allows only slow modulation, but for a large range of wavelength
A need exists for a technique to modulate the linear polarization of a light source operable at high frequency (bandwidth) and over a broad range of wavelengths.